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PYRIMIDINES – One of the 3 isomer forms of
Some properties of nitrogenous bases
- Weak Bases: Purines and Pyrimidines with
NH2 in their structure act as weak bases.
- Planar character: facilitates the close
association or stacking that stabilizes thew
structure of nucleic acids.
• Heterocyclic rings of nitrogenous bases with oxo
groups exhibit keto(Lactam) enol(lactim)
PROPERTY PURINE PYRIMIDINE
Numbering Counterclockwise Clockwise
linkage with the
N present on 9th
N present on 1st position
forms glycosidic linkage.
Biosynthesis Cytoplasm Cytoplasm+Mitochondria
End product of
Uric acid CO2, NH3
• DNA and RNA contain small quantities of
Minor/modified bases also
- Alteration of atoms.
• Modification of Adenine:
• Modification of Guanine:
• Modification of Cytosine:
• Modification of Uracil:
• Special Bases:
Uric acid (2,6,8-trioxopurine)
Purine bases of plants
• Plants contain certain methylated purines.
- Caffeine (1,3,7-trimethylxanthine):
It is found in coffee, It acts as a stimulant of nervous
- Theophylline (1,3-dimethylxanthine):
Present in tea leaves. It acts as a bronchial smooth
- Ribose (in RNA) and Deoxyribose (in DNA).
- Ribose and deoxyribose predominantly exist in
the cyclic form.
- Derivatives of sugar linked to ring Nitrogen of
Purine or Pyrimidine .
- Linkage is formed by -N-glycosidic bond.
SUGAR N9 of PURINE
SUGAR N1 of PYRIMIDINE
• Mononucleotides are Nucleosides with a
phosphoryl group esterified to a hydroxyl
group of the sugar.
• Additional phosphoryl groups, ligated by acid
anhydride bonds to the phosphoryl group of a
mononucleotide, form nucleoside
diphosphates and triphosphates.
• Nucleoside bases common in DNA and RNA are highly
conjugated molecules, a property with important
consequences for the structure, electron
distribution,and light absorption of nucleic acids.
- Polyfunctional acids.
- Absorption of UV light.
- Syn and Anti Conformers.
2.1 PROPERTIES OF NUCLEOTIDES
1. POLYFUNCTIONAL ACIDS
• Phosphoryl groups of nucleosides have pKa values of
1.0, thus contribute to the negative charge at
• pKa values of the secondary phosphoryl groups are
about 6.2 → proton donors or acceptors at pH
values approximately 2 units ± NEUTRAL pH
2. ABSORB U-V LIGHT
• Conjugated double bonds of the purine and
pyrimidine bases are characterized by a strong
absorption at wavelengths near 260 nm at neutral
• Concentration is expressed as molar absorptivity at
- Helps in identification of nucleic acids.
-The mutagenic effect of ultraviolet light is due
to its absorption by nucleotides in DNA that results
in chemical modifications
3. SYN AND ANTI CONFORMERS
• Due to Steric hindrance by the heterocyclic ring there
is no freedom of rotation about the -N-glycosidic
• Both therefore exist as non inter convertible Syn or
Anti conformers .
• Anti Conformers have more common occurrence
• SYN & ANTI CONFORMERS interconversion
- cleavage &
reformation of the glycosidic bond.
i) Principal biologic transducer of free energy in
metabolic inter conversions – ATP.
ATP ADP + Pi
- Group Transfer Potential: ATP acts a donor of
ATP + Creatinine Creatinine-P + ADP
- Formation of a high energy intermediate:
PPi – PYROPHOSPHATE.
ATP AMP + PPi
2.2 NUCLEOTIDES – DIVERSE FUNCTIONS
ii) Allosteric regulation and energy source for
iii) Second messenger for Hormones:
- c-AMP for Glucagon
- c-GMP for Nitric oxide in smooth muscle
iv) Carbohydrate metabolism:
UDP-glucose and UDP- galactose participate in
sugar inter conversions and in the biosynthesis
of - Starch, glycogen
- Glucosyl disaccharides,
- Oligosaccharides of glycoproteins
UDP-Glucuronic acid forms the urinary
glucuronide conjugates of bilirubin and of
many drugs like aspirin
v) CTP participates in biosynthesis of:
- Substituted sphingosines.
vi) Lipid metabolism: Nucleoside-lipid
derivatives such as CDP-acyl glycerol are
intermediates in lipid biosynthesis.
vii) Signal transduction pathways:
- GTP and GDP play key roles in signal
- When linked to vitamins or vitamin
derivatives function as coenzymes
e.g: Niacin, Pyridoxine, Pantothenic acid
derivatives, Coenzyme A.
ix) Donor of various groups:
a) 3'Phosphoadenosine -5'-phosphosulfate
- Sulphate donor for proteoglycans and
sulfate conjugate of drugs.
- Methyl donor in various metabolic reactions in
2.3 SYNTHETIC NUCLEOTIDE ANALOGS
• Modifications in Heterocyclic ring or in the
Sugar moiety SYNTHETIC ANALOG.
nucleic acids with
resulting disruption of
Inhibition of enzymes
essential for nucleic -
1) Cancer chemotherapy:
Incorporated into DNA prior to cell division.
5-fluoro- or 5-iodouracil,
APPLICATIONS OF SYNTHETIC ANALOGS
2) Suppresion of Organ transplant rejection:
3) Viral Diaseases:
- Arabynosyladenoisine for Encephalitis.
- Zidovudine for HIV infection.
4) Metabolic diseases:
- It is structurally similar to hypoxanthine,
inhibits the enzyme activity of XANTHINE
- Used in treatment of hyper uricemia and
POLYNUCLEOTIDE CHAINBASE SEQUENCE OF